This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI with fat-water separation.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
FIG. 5A is a perspective view partially in section illustrating conventional coil apparatus in an NMR imaging system, and FIGS. 5B-5D illustrate field gradients which can be produced in the apparatus of FIG. 5A. This apparatus is discussed by Hinshaw and Lent in “An Introduction to NMR Imaging: From the Block Equation to the Imaging Equation,” Proceedings of the IEEE, Vol. 71, No. 3, March 1983, pp. 338-350. Briefly, the uniform static field B0 is generated by the magnet comprising the coil pair 10. A gradient field G(x) is generated by a complex gradient coil set which can be wound on the cylinder 12. An RF field B1 is generated by a saddle coil 14. A patient undergoing imaging would be positioned within the saddle coil 14.
In FIG. 5B an X gradient field is shown which is parallel to the static field B0 and varies linearly with distance along the X axis but ideally does not vary with distance along the Y or Z axes. FIGS. 5C and 5D are similar representations of the Y gradient and Z gradient fields, respectively.
FIG. 6 is a functional block diagram of conventional imaging apparatus. A computer 20 is programmed to control the operation of the NMR apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22, and the RF coils 26 for impressing an RF magnetic moment at the Larmor frequency are controlled by the transmitter 24. After the selected nuclei have been flipped, the RF coil 26 is employed to detect the FID signal which is passed to the receiver 28 and thence through digitizer 30 for processing by computer 20.
Imaging and diagnosis of articular cartilage abnormalities have become increasingly important in the setting of an aging population where ostcoarthritis is second only to cardiovascular disease as a cause of chronic disability. Accurate assessment of articular cartilage is also essential with the advent of surgical and pharmacological therapies that require advanced imaging techniques for initial diagnosis and management of disease progression.
Ideal imaging of articular cartilage requires high resolution and good contrast with adjacent tissues; this can be markedly improved with fat suppression techniques. In addition, bright appearance of synovial fluid is advantageous as it provides an arthroscopic effect that “fills in” defects in articular cartilage, increasing the conspicuity of cartilage irregularities. Finally, a new sequence for imaging articular cartilage should require short scan times, adding little to the time required for a standard knee protocol.
Steady-state free precession (SSFP) is a rapid gradient echo imaging technique with renewed interest in recent years, owing to widespread availability of high speed gradient systems. SSFP has superior signal to noise ratio (SNR) compared with other gradient echo techniques and has excellent contrast behavior that has mixed dependence on T1 and T2. In particular, synovial fluid appears bright on SSFP images owing to its long T2. The major limitation of SSFP is severe image degradation caused by local magnetic field inhomogeneties if the repetition time (TR) is long.